The invention relates to improvements in riveting tools, and more particularly to improvements in tools for setting so-called blind rivets.
European Pat. No. 0 116 954, granted May 6, 1987 to Totsu for "A riveting motor tool", discloses a power tool which can set blind rivets and employs an electric motor serving to rotate a pinion having three equidistant teeth and meshing with a toothed rack having a single tooth. When the pinion is driven, one of its teeth engages the tooth of the rack and pushes the rack rearwardly whereby the rack entrains a gripper which engages the mandrel of a blind rivet and causes the mandrel to break, i.e., to become severed from the so-called bulge portion or head at the front end of an intact mandrel. The tooth of the pinion becomes disengaged from the tooth of the rack upon rotation of the pinion through a predetermined angle so that a coil spring is free to propel the rack and the gripper back to their starting or extended positions.
The power tool of Totsu exhibits a number of drawbacks. First of all, the ratio of the rack-and-pinion transmission between the motor and the gripper is dependent upon the starting axial position of the rack. In fact, the ratio is least satisfactory at the time the pinion is in the process of severing the mandrel of an applied blind rivet through the medium of the rack and gripper, i.e., at a time when the rack is to transmit a maximal pulling force in order to break or sever the mandrel. The reason is that the lever arm of the transmission is least satisfactory in that axial position of the rack when the gripper is supposed to sever the mandrel. This renders it necessary to ensure that the motor can apply a pronounced torque with attendant consumption of substantial amounts of electrical energy. Furthermore, the motor must overcome the resistance of the mandrel to severing by the gripper as well as the resistance of the spring which is used to bias the rack and the gripper back to their starting or extended positions. In other words, the spring stores energy while the motor is operated in a direction to move the gripper from its extended position for the purpose of severing a mandrel. The resistance of the spring to further deformation by the rack increases as the gripper moves in a direction to sever the mandrel.
If the mandrel does not break before a tooth of the pinion becomes disengaged from the single tooth of the rack, the disengagement of the pinion from the rack results in abrupt propulsion of the rack and gripper back to their starting positions with attendant pronounced wear upon such parts and upon the guide means therefor. In addition, the mating teeth of the pinion and the rack also undergo extensive wear. Abrupt disengagement of the pinion from the rack results in the generation of a pronounced shock which must be taken up by the hand holding the housing of the patented power tool.
A further drawback of the patented power tool is that the rack must complete a full rearward stroke before it becomes disengaged from the pinion, irrespective of whether the mandrel breaks during an initial stage or during a later stage of movement of the rack from its extended position. The rack must reassume the starting position in order to maintain the gripper in a position in which the gripper is ready to receive the mandrel of a fresh blind rivet. Any rearward movement of the rack which takes place upon completed severing of a mandrel is undesirable because the motor must consume considerable amounts of energy (the motor must then rotate the pinion against the resistance of the progressively stressed spring). Abrupt propulsion of the rack and gripper back to their extended positions upon disengagement of the pinion from the rack (i.e., after the rack has completed a full rearward stroke) results in the generation of readily detectable noise and causes shaking and other stray movements of the housing of the patented power tool.
The power tool of Totsu is operated by a motor which receives energy from an accumulator or from a battery. If the supply of energy in the battery or in the accumulator is nearly exhausted so that the motor is incapable of causing the rack and the gripper to sever a mandrel, a specific embodiment of the patented tool is designed to employ a reversible electric motor which can move the rack and the gripper back to their extended positions in which the gripper can be disengaged from the still intact mandrel.
U.S. Pat. Nos. 3,375,883 and 3,127,045 disclose power tools for the application of blind rivets wherein the transmission which converts rotary movements of the output element of an electric motor into translatory movements of the gripper for the mandrel of a blind rivet comprises an eccentric and a connecting rod. A clutch is provided to connect the continuously operated electric motor with the connecting rod. A drawback of such power tools is that one-half of each rearward stroke of the gripper must take place under highly unsatisfactory circumstances, namely when the lever arm of the transmission is least satisfactory for transmission of motion to the gripper. The mandrel is to be severed during such least satisfactory stage of movement of the connecting rod so that the power tool must employ a strong motor whose energy requirements are high. This ensures that the gripper will be in a position to sever an engaged mandrel irrespective of the momentary condition of the transmission including the eccentric and the connecting rod. The motor is bulky and contributes significantly to the weight of the patented power tool. This, in turn, necessitates the exertion of a considerable effort by the operator of the power tool so that the operator is likely to be exhausted after a relatively short period of use of the patented tool.
U.S. Pat. No. 3,095,106 granted Jun. 25, 1963 to Morrison discloses an automatic rivet setting tool wherein the prime mover is an electric motor which can drive the gripper for the mandrel of a blind rivet through a gear train and a ball screw transmission. The prime mover transmits torque to the feed screw of the transmission. A spring is employed to return the gripper to its starting position upon disengagement of a clutch. Thus, the gripper receives motion from the motor or from the screw.
U.S. Pat. No. 4,085,337 granted Apr. 18, 1978 to Moeller for "Electric drill multi-functional apparatus" discloses an apparatus for setting self-cutting rivets and for performing additional functions including drilling holes for rivets and hammering the rivets. The chuck which drives the rivets must rotate about its axis and must move back and forth in order to drive the mandrel of a rivet into one or more workpieces. The electric motor of the patented apparatus is connected to an outside source of electrical energy by a cable.